Abstracts

Patients with epilepsy suffer from varying degrees of postictal impairment including confusion and amnesia. This impairment adds substantially to the disease burden of epilepsy. However, the mechanism responsible for postictal cognitive impairment is unclear. In this study we monitored place cells in the CA1 region of the hippocampus before and after spontaneous seizures in epileptic rats. Place cells fire action potentials when the animal is in a specific location in space, the so-called place field. Place cell function correlates well with performance in tasks of visual-spatial memory and are an excellent measure of spatial memory., Male Sprague-Dawley rats at P70 were given lithium chloride (127mg/kg) given intraperitoneally (IP) 18 hours before an IP injection of pilocarpine (34 mg/kg) to induce SE . One month later the rats were implanted with eight tetrodes and place cells and interneurons recorded. We compared place cell and interneuron firing patterns before and after spontaneous seizures. For the pyramidal neurons we measured coherence, information content, AP firing , and total number of APs., A total of 12 spontaneous seizures were recorded from six rats. Of 87 cells recorded in these six rats, 20 met the requirements for a place cell and were detected during all four recording sessions (two before and two after the seizure). In these cells there were significant decreases in coherence, information content, firing rate and total spikes following the seizure (Figure). In addition, eight cells that did not have place firing prior to the seizure developed firing fields following the seizure. Nine cells that had firing fields on both recording sessions prior to the seizure could not be recorded following the seizure. Seven interneurons were recorded from four rats were recorded. The seizures had no effect on firing frequency of the interneurons., Following seizures there was a dramatic decrease in firing rate of action potentials. In addition, firing fields were aberrant with reduced coherence and information content. In addition, to postictal suppression of firing patterns, seizures lead to the emergence of previously silent cells. Interneuron firing was not affected by seizures. The concept that postical impairment is due to simple neuronal exhaustion is no longer tenable. Our findings demonstrate that the effects of seizures are complex, affecting neuronal function in a cell specific manner.[figure1], (Supported by Western Massachusetts Epilepsy Awareness Fund, Friends of Shannon McDermott, the Sara fund, and grants from NINDS (Grants: NS27984 and NS44295).)